The present invention relates to a screen printing machine with an improved device for aspirating excess ink.
The screen-printing method is currently widely used to reproduce by application of a printing medium, generically called ink, patterns as images, words, drawings or figures and the like on sheet material such as paper, glass plates, plastic backings or fabrics etcetera.
In particular, this printing method, when used to apply images to glass plates, uses a mesh frame which is arranged above a fixed supporting surface and whereon minute through openings are formed, which, as a whole, form the image to be printed.
The mesh frame is spread with ink (which can be black or colored); the ink can pass only through the openings, thus reaching the item to be printed and fixing thereon so as to form the image.
This printing method is usually used mainly on windshields and on rear and side windows of motor vehicles.
All these components must in fact be screen-printed with borders at their perimetric edges.
The borders are aesthetically indispensable because they conceal the region where each component is assembled in its respective seat.
Currently it is in fact customary to assemble, for example, windshields by arranging them in the complementarily shaped seat of the structure of the vehicle by means of adhesives, without having to first insert therein special weather stripping inside which the seat for the windshield used to be formed.
Currently, therefore, the border of the windshield and of all the other glass parts must conceal the underlying coupling region and accordingly must be printed with maximum precision and accuracy.
Unfortunately, screen-printing machines are unable to work according to the printing process known in the field as "edge to edge" without producing excess ink on the edge of the glass plate.
This excess produces deposits (ink drops) which are not allowed on the edge of the windshield because they dirty the glass.
Machines are currently available which have a mechanical ink scraping device that acts on the mesh frame from below.
This device, however, after a certain number of production cycles, inevitably produces an application of ink on the printing region which causes smudges and shadows.
Other screen-printing machines are commercially available which have scraping devices which operate in combination with pads, for example rolls of paper, by means of which an attempt is made to delete shadows and smudges without however achieving their full elimination.
Finally, there are present machines which are provided with an aspirator with a suction nozzle which acts on the upper part of the mesh frame, i.e., on the part on which the ink is spread.
The aspirator is capable of aspirating ink and any residues thereof on all of the exposed region of the frame by means of the suction nozzle, whose dimensions are at least equal to the width of the frame and which is actuated by a mechanical device which makes it shift along all of the frame.
This last machine, however, is unable to perfectly aspirate only the excess ink on the edge; most of all, it produces uneven image definition on the glass plate.
These machines are in fact designed and built to work in a continuous cycle and to print one glass plate after the other.
The aspirator instead aspirates ink on the frame and in a short time dries the ink between the openings of the frame, making it difficult for new ink to pass through to print a subsequent glass plate.
Accordingly, constant cleaning of the frame is necessary to clear the openings formed in the frame from the clogging produced by the dried ink.